The semiconductor die attachment process is one of the steps involved in semiconductor device manufacturing. It involves attaching a semiconductor die to a specific bond pad on a substrate. The substrate can be in the form of a lead frame, a printed circuit board (PCB) or other suitable mechanical structure. The attachment is usually achieved by first dispensing an adhesive material onto the substrate, and then pressing the die into the adhesive material under a certain pressure and for a certain time.
After the die is attached, a thermal treatment such as oven curing is performed in order to solidify the adhesive and firmly secure the die on the substrate. The secured die is then electrically coupled to the substrate by connecting bonding wires between the die and the conductive leads on the substrate. The die and the bonding wires are finally encapsulated in a protective case using a molding material, such as a thermoplastic resin or ceramic, to complete the packaging of the semiconductor device.
The thickness of adhesive between the bottom surface of the die and the substrate upper surface is referred as the bond line thickness (BLT). In general, the average BLT of the bonded die is a key factor affecting the reliability of the final product of the semiconductor device. If the BLT is too thin, the die may not be completely secured on the substrate. On the other hand, if the BLT is too thick, excess adhesive may overflow to contaminate the die surface. Accordingly, much attention in the art has been focused on controlling the average bond line thickness for the die attachment process.
In addition to controlling the average BLT, it is important to carefully control the die tilt (the maximum difference between the BLT at the four corners of the die), which can also have an impact on the semiconductor device's reliability. For instance, the die tilt may make the stress distribution across the device uneven, with the region with the thinnest bond line experiencing the highest stress. Further, for some devices the die tilt can have a significant effect on product quality for other reasons. For example, for an optical device such as a miniature camera, any tilt of the image sensor die will result in misalignment between the sensor element axis and the optical element axis. This reduces the image quality produced by the optical device.
Conventionally, die tilt is minimised by adjusting the bonding tool tilt to the same level as the die attach platform tilt prior to bonding. However, thermal effects caused by machine operation may change the bond tool tilt level. This can result in a die tilt variation throughout the bonding operation. Substrate surface level variation, which may occur due to variations in the substrate manufacturing process, is another factor that can affect die tilt.
Conventional tilt adjustment processes carried out during bonding can only guarantee that die tilt and BLT are optimal immediately after the die has been attached. The adhesive used to bind the substrate and the die has not been cured at that moment. The bonded substrate is subject to movement while being transferred to subsequent processing steps. The adhesive between the substrate and the die is consequently still subject to change, such that the die tilt and BLT of the bonded sample are not stabilised.
To stabilise the die orientation and BLT, thermal curing of the adhesive must be performed. Previously it has been proposed to perform an instant adhesive cure process during die bonding, by providing a bond tool with an embedded heater (see U.S. Pat. No. 5,971,250). As the bond tool presses the die onto the substrate, the heater in the bond tool can heat up the adhesive to cure it. A disadvantage of this proposal is that embedding a heater in a bond tool complicates the bond tool design. In addition, the direct heating of the bond tool may change the tilt level setup due to thermal effects.
There remains a need for a die attachment tool and system which overcomes or alleviates at least one of the foregoing difficulties, or which at least provides a useful alternative.